Pollution control devices are employed on motor vehicles to control atmospheric pollution. Two types of such devices, catalytic converters and diesel particulate filters or traps, are currently in widespread use. Catalytic converters contain a catalyst, which is typically coated onto a monolithic structure mounted in the converter. The monolithic structures (termed “monoliths”) are typically ceramic, although metal monoliths have been used. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Diesel particulate filters or traps are generally wall flow filters which have honeycombed monolithic structures (also termed “monoliths”), typically made from porous crystalline ceramic material.
Typically, as constructed, each type of these devices has a metal housing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The ceramic monolith generally has very thin walls to provide a large amount of surface area and is fragile and susceptible to breakage. Moreover, there is a current industry trend regarding ceramic monoliths used in automotive catalytic converters toward monoliths having smoother outer surfaces that makes it more difficult to maintain adhesion of the mounting material to the ceramic monolith during canning. Ceramic monoliths also typically have coefficients of thermal expansion that are an order of magnitude less than typical metal housings (for example, stainless steel housings) in which they would be contained in use. To avoid damage to the ceramic monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing, ceramic mat or intumescent sheet materials are often disposed between the ceramic monolith and the metal housing.
During use the leading edge of the mounting material can become eroded by hot exhaust gases, which may result in, for example, mechanical failure of the mounting material or exhaust gases passing through the mounting material and bypassing the monolith, or plugging of the ceramic monolith cells by free vermiculite or fiber fragments.
There are two methods currently in commercial use today to reduce erosion caused by exhaust gas impingement in pollution control devices. In the first method, known in the art as a “soft seal”, an erosion resistant strip of polycrystalline mat is taped or otherwise adhered to the leading peripheral edge of a mounting mat prior to the canning step. The second method involves the application of a rigidizing solution to the leading peripheral edge of a mounting mat during the canning step. Typical application methods include dipping, spraying, or painting the solution onto the leading peripheral edge of the mounting mat, either just prior to inserting the mat wrapped monolith into the stainless steel shell or can, or it is dripped on the desired edge after the mat and monolith have been inserted into the shell or can. This process must be done during the canning process because the rigidizing solution, when dried, stiffens the edge of the support mat to an extent that it can't be wrapped around a monolith without causing the mounting mat to crack as it is being wrapped around the monolith. Wet processes such as this are inherently messy, and are typically undesirable.